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Visual speech speeds up the neural processing of auditory speech
van Wassenhove, V., Grant, K. W., & Poeppel, D. (2005) Proceedings of the National Academy of
Sciences, 102(4), 1181-1186.
Jaimie GilbertPsychology 593October 6, 2005
Audio-Visual Integration
Information from one modality (e.g., visual) can influence the perception of information presented in a different modality (e.g., auditory) Speech in noise McGurk Effect
Demonstration of McGurk Effect
Audiovisual Speech Web-Lab
http://www.faculty.ucr.edu/~rosenblu/lab-index.html
Arnt Maasø University of Oslo
http://www.media.uio.no/personer/arntm/McGurk_english.html
Unresolved questions about AV integration
Behavioral evidence exists for vision altering the perception of speech, but…
When does it occur in processing?
How does it occur?
ERPs can help answer the “when” question
EEG/MEG studies have demonstrated AV integration effects using oddball/mismatch paradigms These effects occur around 150-250 ms
A non-speech ERP study with non-ecologically valid stimuli demonstrated earlier interaction effects (40-95 ms) (Giard & Peronnet, 1999)
Does AV integration for speech occur earlier than 150-250 ms?
There’s a debate about the “how” question…
Enhancement Audio-visual integration generates activity
at multi-sensory integration sites, information possibly fed back to sensory cortices
VS. Suppression
Reduction of stimulus uncertainty by two corresponding sensory stimuli reduces the amount of processing required
The Experiments
3 experiments were conducted Each had behavioral and EEG measures Behavioral: Forced choice task EEG: Auditory P1/N1/P2
26 participants Experiment 1: 16 Experiment 2: 10 Experiment 3: 10 (of the 16 who
participated in Experiment 1)
The Stimuli
Audio /pa/ Audio /ta/ Audio /ka/ Visual /pa/ Visual /ta/ Visual /ka/ AV /pa/ AV /ta/ AV /ka/ Incongruent AV with
Audio /pa/ + Visual /ka/
1 Female face & voice for all stimuli
In Exp. 1 & 2, each stimuli presented 100 times; total of 1000 trials
Experiment 1
Exp. 1 Stimuli presented in blocks of
audio, or blocks of visual, or blocks of AV (congruent and incongruent)
Participants knew before each block which stimuli were going to be presented
Experiment 2
Exp. 2 Stimuli presented in randomized blocks
containing all stimuli types (A, V, Congruent AV, Incongruent AV) to reduce expectancy
Task for both experiments: choose which stimuli was presented; for AV--choose what was heard while looking at the face
Experiment 3
Presented 200 Incongruent AV stimuli
Task: choose what syllable you saw, neglect what you heard
In all experiments, correct response to Incongruent AV = /ta/
Waveform Analysis
Retained 75-80% of recordings after Artifact Rejection and Ocular Artifact Reduction
Only correct responses were analyzed 6 electrodes used in analysis: FC3, FC4,
FCz, CPz, P7, P8 Reference electrodes: Linked mastoids
Results
This study’s answer to “How” Suppression/Deactivation Hypothesis
AV N1 & P2 amplitude were significantly reduced compared to Auditory-alone peaks
Performed separate analysis to determine if summing the responses to unimodal stimuli would result in the amplitude reduction present in the data—this was not the case; therefore the AV waveform is not a superposition of the 2 sensory waveforms, but reflects actual multisensory interaction.
Results: Experiment 1
N1/P2 Amplitude AV < A (p < .0001)
N1/P2 Latency AV < A (significant, but confounded
by interaction) Modality x Stimulus Identity
P < T < K (p < .0001) Latency effect more pronounced in
P2, but can occur as early as N1
Results: Experiment 2
N1/P2 Amplitude AV < A (p < .0001)
N1/P2 Latency AV < A (p < .0001) Modality x Stimulus Identity (p < .06)
Results: comparison of Exp. 1 & Exp. 2
Similar results for Exp. 1 & 2; Temporal facilitation varied by
Stimulus Identity but amplitude reduction did not;
No evidence for attention effect (i.e., for expectancy affecting waveform morphology)
Temporal facilitation depends on visual saliency/signal redundancy
More temporal facilitation is expected to occur if: The audio and the visual signals are
redundant The visual cue (which naturally
precedes the auditory cue) is more salient
(Figure 3)
Results: Experiment 3/Incongruent AV Stimuli
Incongruent AV stimuli in Exp. 1 & 2: no temporal facilitation Amplitude reduction present and
equivalent to reduction seen for Congruent AV stimuli
Experiment 3: Both temporal facilitation and
amplitude reduction occurred
Visual speech effects on auditory speech
Perceptual ambiguity/salience of visual speech affects processing time of auditory speech
Incorporating visual speech with auditory speech reduces the amplitude of N1/P2 “independent of AV congruency, participant’s expectancy, and attended modality” (p. 1184)
Ecologically valid stimuli
Suggest that AV speech processing is different from general multisensory integration due to the ecological validity of speech
Possible explanation for amplitude reduction
Visemes provide information regarding place of articulation
If this information is salient and/or redundant with auditory place of articulation cues (e.g., 2nd and 3rd formants), the auditory cortex does not need to analyze these frequency regions, resulting in fewer firing neurons
Analysis-by-Synthesis Model of AV Speech
Perception Visual speech activates internal
representation/prediction This representation/prediction is updated as
more visual information is received over time Representation/prediction is compared to the
incoming auditory signal Residual errors to this matching process are
reflected by temporal facilitation and amplitude reduction effects
Attended modality can influence temporal facilitation
Suggest 2 time scales for AV integration
1: feature stage 25 ms Latency facilitation (sub-)segmental analysis
2: perceptual unit stage 200 ms Amplitude reduction Syllable level analysis Independent of feature content and
attended modality
Summary
AV speech interaction occurs by the time N1 is elicited (50-100 ms)
Processing time of auditory speech varies by the saliency/ambiguity of visual speech
Amplitude of AV ERP reduced when compared to amplitude of A-alone ERP
Questions
Dynamic visual stimulus and ocular artifact
If effects of AV integration are influenced by attended modality, would modality dominance also influence these effects?
Are incongruent AV/McGurk stimuli ecologically valid?
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